1. Field of the Invention
The present invention relates to systems and methods for estimating the carrier to noise ratio (CNR) from received signals, particularly signals using layered modulations.
2. Description of the Related Art
Digital signal communication systems have been used in various fields, including digital TV signal transmission, either terrestrial or satellite. As the various digital signal communication systems and services evolve, there is a burgeoning demand for increased data throughput and added services. However, it is more difficult to implement either improvement in old systems and new services when it is necessary to replace existing legacy hardware, such as transmitters and receivers. New systems and services are advantaged when they can utilize existing legacy hardware. In the realm of wireless communications, this principle is further highlighted by the limited availability of electromagnetic spectrum. Thus, it is not possible (or at least not practical) to merely transmit enhanced or additional data at a new frequency.
The conventional method of increasing spectral capacity is to move to a higher-order modulation, such as from quadrature phase shift keying (QPSK) to eight phase shift keying (8PSK) or sixteen quadrature amplitude modulation (16QAM). Unfortunately, QPSK receivers cannot demodulate conventional 8PSK or 16QAM signals. As a result, legacy customers with QPSK receivers must upgrade their receivers in order to continue to receive any signals transmitted with an 8PSK or 16QAM modulation.
It is advantageous for systems and methods of transmitting signals to accommodate enhanced and increased data throughput without requiring additional frequency. In addition, it is advantageous for enhanced and increased throughput signals for new receivers to be backwards compatible with legacy receivers. There is further an advantage for systems and methods which allow transmission signals to be upgraded from a source separate from the legacy transmitter.
It has been proposed that a layered modulation signal, transmitting non-coherently both upper and lower layer signals, can be employed to meet these needs. See Utility application Ser. No. 09/844,401. In backwards compatible implementations, the lower layer signal is transparent or “invisible” to the upper layer signal, the primary signal distribution layer, thereby providing backward compatibility with legacy satellite receivers. Such layered modulation systems allow higher information throughput with backwards compatibility. However, even when backward compatibility is not required (such as with an entirely new system), layered modulation can still be advantageous because it requires a TWTA peak power significantly lower than that for a conventional 8PSK or 16QAM modulation format for a given throughput.
However, to receive such layered modulation signals requires reconstruction of the upper layer signals to remove them from the total signal for lower layer signal processing to occur. Minimizing the CNR is clearly an important factor in producing usable layered signals. CNR degradation occurs primarily as a consequence of receiver thermal noise and satellite traveling wave tube amplifier (TWTA) non-linearity. An accurate measurement of the CNR is an important component of an operational layered modulation scheme. Previously, CNR measurement has been performed by first demodulating and FEC decoding the received signal based upon an analog to digital (A/D) signal at base-band. However, layered modulation and other systems would be advantaged by techniques which measure CNR without FEC decoding and even without completely demodulating the signal. This has the advantage of speeding up the CNR estimation process for both on-line real time systems and off-line computer processing environments.
Accordingly, there is a need for systems and methods that enable CNR measurement without first demodulating the received signal based upon an analog to digital (A/D) signal at base-band. The present invention meets these needs.